In the default template of button, there is trigger in ControlTemplate which set Background of button to #FFBEE6FD and since control template triggers have higher precedence compared to Style triggers that's why your trigger never works.

For that to achieve you have to override default template of button and remove that trigger from it so that your style trigger gets applied.

Here is the default template with that specific trigger commented out. In case you want to override BorderBrush as well, get rid of it as well from the template.